Conventionally, semiconductor devices in the form of semiconductor dice or integrated circuits are housed into packages. A package serves various important functions such as protecting the device from mechanical and chemical damage. It is also a bridge that interconnects the device with a next level of packaging. Die attachment is one of the steps involved in the packaging process during which the die is placed on and attached to a die pad formed on the carrier or substrate. There are various methods for attaching the device onto the die pad, such as by using epoxy and adhesive resin as an adhesive to stick the device onto the pad or stamping flux on the pad and placing a die with solder on its back surface onto the flux before performing a solder reflow process.
An increasingly popular approach is to directly mount a die with a back surface of the die coated with solder onto a heated substrate. The solder melts when it comes into contact with the heated substrate, and a bond is formed to the substrate. This method is conventionally termed as eutectic die bonding, since the solder on the die is usually made from a composition of eutectic alloy.
Eutectic die bonding takes advantage of the lower melting point of eutectic alloys as compared to pure metals. The temperature of the substrate should be raised to above the melting point of the solder on the back surface of the die so that the solder melts immediately when the device is in contact with the die pad. When the substrate is subsequently cooled down, a metallurgical bond will form between the back surface of the die and the pad on the substrate. Some advantages of eutectic bonding over epoxy bonding include a higher service temperature capability for the die, good thermal/electrical conductivity between the die and the substrate and higher reliability.
FIG. 1 is a side view of a prior art die bonding apparatus 100 which is in a standby condition. The die bonding apparatus 100 generally comprises two main components, namely a bond arm support 102 and a bond arm 104. The bond arm 104 is linked to the bond arm support 102 via a sliding mechanism, so that the bond arm 104 is movable relative to the bond arm support 102. A collet 106 is mounted onto the bond arm 104 for holding dice, usually by utilizing vacuum suction, and bonding them onto bonding surfaces.
The bond arm support 102 is driven to undergo up-and-down z motion 108 to move the bond arm 104 and collet 106 towards or away from a bonding surface. The bond arm 104 is preferably preloaded to exert a downwards bonding pressure via the collet 106 onto a die positioned on a bonding surface, but is configured for upwards z motion 110 when the collet 106 is in contact with a relatively rigid surface that overcomes the preload force. The bond arm 104 will start to move upward 110 relative to the bond arm support 102 when the collet 106 contacts the rigid bonding surface and the bond arm support 102 is moved further towards the bonding surface.
The die bonding apparatus 100 also includes a contact sensor 112 that has separate components that are respectively mounted onto the bond arm support 102 and the bond arm 104. The separate components of the contact sensor 112 are touching at a standby position of the die bonding apparatus 100. Once the die held by the collet 106 contacts the rigid surface and the bond arm 104 undergoes upward movement 110 relative to the bond arm support 102, the components of the contact sensor 112 separate and contact is detected.
FIG. 2 is a side view of the prior art die bonding apparatus 100 of FIG. 1 when it is performing bonding. The collet 106 is holding a die 114 which is to be bonded onto a bonding surface 116. The bond arm support 102 lowers the bond arm 104 towards a pre-set bonding level and the die 114 lands on the bonding surface. Upon the die 114 contacting the rigid bonding surface, the components of the contact sensor 112 are separated and contact of the die with the bonding surface 116 is sensed. After the die 114 contacts the bonding surface 116, the bond arm support 102 will be moved slightly further towards the bonding surface 116 in order to utilize the preload in the bond arm 104 to apply a bond force onto the die 114. The die 114 is bonded onto the bonding surface 116 with such bond force. It should be appreciated that the separation distance of the components of the contact sensor 112 has been exaggerated in FIG. 2 for illustration purposes only.
Present die bonding apparatus 100 utilize open loop systems, which have no feedback control to enable bonding level adjustment during bonding. “Bonding level” refers to the level at which the bond arm support 102 is positioned during bonding so that a die 114 is made to contact the bonding surface 116 with a predetermined bonding force during bonding. The bonding level is only adjusted once when setting up the die bonding apparatus 100 for bonding. After the bonding level is decided and set, it will not be re-adjusted throughout the continuous running of the die bonding apparatus 100.
However, particularly for eutectic die bonding where heat is utilized to melt the solder on the die, there is usually a gradual change of the collet tip level with respect to the pre-set bonding level, due to factors such as the thermal expansion of the collet body or the wearing out of the collet tip. If the bonding level is kept constant, different collet tip levels lead to different approaching speeds of the collet 106 towards the bonding surface 116 of the substrate during continuous operation, and this causes different impact forces acting on the die 114, and the collet 106 itself, during bonding.
FIG. 3 is a graph showing a height of the bond arm support 102 over time as it moves to bond a die to a bonding surface 116. In the graph, the height of the bond arm support 102 relative to the bonding surface 116 decreases as the collet 106 is lowering a die 114 onto the bonding surface 116. A speed of approach of the bond arm support 102 decreases as the collet 106 nears the bonding surface 116, as represented by the decreasing slope of the graph. At contact point 118, the die 114 is in contact with the bonding surface 116. Z drive-in is then performed to further lower the bond arm support 102, say by another ΔD, in order to exert a bonding force onto the die 114.
For conventional die bonding apparatus 100, if the physical characteristics of the collet 106 change, such as if the body of the collet 106 expands, then the preset bonding level would no longer be optimal. For example, if the collet 106 expands by ΔL due to thermal expansion, according to FIG. 3, the die 114 would instead contact the bonding surface at contact point 120. At contact point 120, the approach speed of the bond arm support 102 is faster than at contact point 118. This implies that the impact force is greater when the die 114 contacts the bonding surface 116 and this increases the risk of damage to the collet 106 and/or die 114.
Furthermore, since the collet 106 has expanded by ΔL, the actual Z drive-in is increased to ΔD+ΔL, which exerts a greater bonding force than was intended when setting up the apparatus 100. This may adversely affect the bonding quality, and it is also difficult to anticipate the actual amount of deviation ΔL to compensate for this.
Thus, it is a shortcoming of such prior art die bonding apparatus that undesirable deviations in the impact forces may lead to collet and/or die damage, which might further lead to bonding stoppages due to clogging of the collet 106 or missing dice. These lead to reduced production throughput.
Large impact forces also lead to a shorter collet lifespan, which requires replacing collets more often during production. Again, this affects the production throughput, and increases the production cost. Moreover, uncontrolled variations of the effective position of the collet tip during continuous bonding causes issues such as varying and unpredictable bonding quality, wetting, weakened die shear strength, and even cracked dice.